1. Field of the Invention
The present invention relates to an image reader, an image forming apparatus and a method of correcting output values and more specifically, to an image reader of a duplex scanning type that reads both sides of a target to be read, an image forming apparatus including such an image reader and a method of correcting output values output in such an image reader.
2. Description of the Related Art
As for a duplex scanning type image reader, a first reading unit (a reading unit for reading an image on a front surface of a document, for example) having a movable reading section (a movable mirror unit or the like) and a second reading unit of a fixed contact type (a reading unit for reading an image on a back surface of the document, for example) are used as reading means in many cases. In such a case, because of the reason of transfer of the document or generation of shading data, different background members are used for the first reading unit and the second reading unit. For example, there is a case where a guide board or a transfer belt is used for the background member of the first reading unit and a white board roller is used for the background member of the second reading unit.
However, there exists a difference in shading between read images respectively read by the first reading unit and the second reading unit even when a same document is read when the different background members are used for the first reading unit and the second reading unit because of the influence of the background members when reading the document. Especially, for an achromatic document, there is a problem that the images seem to have different colors even with slightly different shades.
As for such a problem, it is disclosed in Japanese Laid-open Patent Publication NO. 2008-187466, for example, that an image reader itself outputs a color correction chart of total colors or a specified color as a reference, and corrects colors of read images of both sides read by a first reading means for reading a front surface of a document and a second reading means for reading a back surface of a document based on read images obtained by reading the color correction chart by the first reading means and the second reading means.
However, when the document is thin, the influence of the background members becomes large while when the document is thick, the influence is not large. According to the technique disclosed in Japanese Laid-open Patent Publication No. 2008-187466, such a difference in thickness of the read document is not considered, and when the color chart is formed on a thin document, read images respectively read by the first reading means and the second reading means have difference in shading because of the influence of the background members and appropriate correction cannot be performed.